Electrical Properties Of Earth Research Articles

Attitude corrections of helicopter EM data using a superposed dipole model

The analysis of helicopter‐borne electromagnetic (EM) survey data commonly assumes that the EM sensor (bird) has flown straight and levelly, whereupon the coils are either horizontal or vertical. The EM data typically are transformed, under this assumption, to yield the electrical properties (conductivity, permeability, permittivity) of a homogeneous earth, or they are inverted to a horizontally layered earth. In actual fact, the bird exhibits some roll, pitch, and yaw rotation, which will generate changes to the data that we call bird attitude effect. On the basis of a superposed dipole model, we distinguish the geometric part of the bird attitude effect from the inductive part. Theoretical investigation shows that the geometric effect results from the reorientation of the bird's coil axes related to the earth. It is independent of the system frequency and the electrical properties of the earth but dependent only on the bird's attitude. In contrast, the inductive effect results from the coupling change between the coils and the conductive earth. It depends not only on the attitude of the bird but also on the system frequency and the earth's electrical properties. We calculate ratios of the EM responses when the bird is rotated to those when the bird flies straight and levelly. These ratios, reflecting the purely geometric effect of the bird's rotation, are simple trigonometric functions of the rotation angles and thus can be easily incorporated into our processing of the EM data. We show that, in the general case of helicopter EM systems, the majority (>95%) of the attitude error results from the geometric effect rather than from the inductive effect. Sufficient accuracy is obtained by using the simple geometric technique for attitude correction, without requiring information about the earth's electrical properties. We demonstrate the usefulness of the method, using an example from a test survey.

Role of electrical properties in airborne and satellite borne sensing

Abstract The sensitivity of airborne and satellite borne electromagnetic surveys depends on the changes in the electrical properties of earth. These electrical properties vary significantly with various physical parameters in the extended frequency of electromagnetic signals. In the present paper, the effectiveness of electrical properties in the airborne and satellite borne electromagnetic surveys have been discussed.

The mechanical and electrical properties of earth's asthenosphere

An adequate theory of continental drift can be based on heat transfer theory, but it does demand the acceptance of a large downward revision of traditional estimates of average upper mantle temperatures and a consistent understanding of lithosphere and asthenosphere in terms of a difference in rheological behaviour under prolonged non-hydrostatic stress. The recognition that an extremely viscous average state of the upper mantle is self regulating both requires and permits an explanation of magma generation at a strictly limited rate (when averaged for the whole planet over a few years) in terms of unsteady and local deformational heating. The activity of water as a reducer of silicate creep resistance is used to develop the hypothesis that water produced by an amphibole dehydration has been effectively trapped in the Earth and is the underlying cause of a low seismic Q ∼ 50 and an electrical conductivity 10 −2 −10 −1 ohm −1 m −1, at depths of ∼ 100 km. At the predicted low horizontally-avera temperatures, the conductivity contrast of rock and aqueous solutions is very large, and mantle electrical conductivity studies now look best-suited to explore this trapping process, and the distinctly recognisable possibility that the uptake of ocean water in the subduction process exceeds the rate of loss that can be explained purely through magmatic activity.